1. Technical Field
The present invention relates to an imaging device that is equipped with a solid state imaging element containing a plurality of pixels that have a photoelectric converting element, which has a pair of electrodes stacked over a semiconductor substrate and a photoelectric converting layer put between the electrodes, respectively.
2. Description of the Related Art
In JP-A-2005-268477, the solid state imaging element equipped with a semiconductor substrate on a signal reading circuit composed of MOS transistor circuits is formed, a photoelectric converting film stacked on the semiconductor substrate to generate charges in response to a quantity of incident light, a connecting portion which is provided on a surface of the semiconductor substrate and to which a wiring for guiding the charges to the surface of the semiconductor substrate is connected, a potential barrier unit provided in vicinity of the connecting portion and acting as a predetermined potential barrier against the charges, and a charge accumulating portion provided in vicinity of the potential barrier unit and connected to a gate of an output transistor constituting the signal reading circuit is disclosed.
According to this structure, the charges generated by a photoelectric converting film can be flown smoothly to the gate of the output transistor. Therefore, an effective electrostatic capacity of a portion in which the charges are accumulated can be decreased when viewed from the gate of the output transistor, a signal voltage can be increased and thus an output signal of high sensitivity can be obtained.
FIG. 10 is a view explaining an operation of the solid state imaging element disclosed in JP-A-2005-268477, and is a view showing a sectional potential in the semiconductor substrate.
In order to acquire a signal from one pixel of the solid state imaging element, first the charges accumulated in the charge accumulating portion are discharged to the reset drain and the reset operation is executed (time T0). At this time, as shown in FIG. 10, a reset noise N1 as noise charges generated due to the reset operation is accumulated in the charge accumulating portion. After the reset is completed, the exposure of the photoelectric converting film is started, and then the charges Q generated by this exposure are accumulated in the charge accumulating portion from the connecting portion through a potential barrier (time T1). Then, a signal responding to a quantity of charge accumulated in the charge accumulating portion during this exposure period is output from the signal reading circuit. After this signal output, the reset operation is executed again at a time T2, and a reset noise N2 is accumulated in the charge accumulating portion. Then, the next exposure is started in this state.
In the signal processing circuit for processing the signal output from the solid state imaging element, the correlated double sampling (CDS) circuit for applying a correlated double sampling process to the signal is contained. In this CDS circuit, sampling of both a signal responding to the reset noise and a signal responding to the charges in which the reset noise is contained are executed, and then the reset noise is removed by calculating a difference between both signals.
The reset noise can be removed completely by subtracting the signal responding to the reset noise N1 acquired at a time T0 from the imaging signal acquired at a time T1. In order to make such process, in the CDS circuit, the signal being output from the solid state imaging element must be sampled at a time T0 (sampling SP1), then the signal being output from the solid state imaging element must be sampled at a time T1 (sampling SP2), and then a difference between both resultant signals must be calculated. In this case, a time period from the sampling SP1 to the sampling SP2 is equal to an exposure period, and the CDS circuit has to process sequentially the signals from all pixels. Therefore, if the samplings of signals are executed at such time interval, the process cannot catch up with the signals that are being output from the solid state imaging element sequentially.
For this reason, in the prior art, removal of the reset noise (a part of reset noise can be removed, although such noise cannot be perfectly removed) is executed by subtracting the signal that is sampled at a time T2 (sampling SP3) from the signal that is sampled at a time T1. Since a time interval between the sampling SP2 and the sampling SP3 is sufficiently short in contrast to the exposure period, the CDS process can be applied to the signals from all pixels without any trouble.
The charges that decide a potential of the connecting portion always flow out from the connecting portion to the charge accumulating portion due to thermal diffusion. In the meanwhile, members such as wirings, photoelectric converting film, and the like are connected to the connecting portion. Thus, the charges generated in the photoelectric converting film flow into the connecting portion via the members, and the charges generated in a joined portion between the connecting portion and the substrate flow into the connecting portion. Therefore, an outflow of the charges from the connecting portion due to the thermal diffusion stops apparently when a quantity of charges per unit time that flow out from the connecting portion due to the thermal diffusion becomes equal to a quantity of charges per unit time that flow into the connecting portion via the wiring, etc. (this moment is called an equilibrium state).
A total quantity ΔQ of charges that are diffused from the connecting portion to peripheral areas due to thermal excitation until a time t (<time teq at which the charges reach an equilibrium state) is given by following Expression (1).ΔQ∝1n (1+αt)   (1)
Where α is a proportionality factor.
As shown at a time T0 in FIG. 9, such a situation is ideal that, as the result of this equilibrium state, the potential barrier and the connecting portion are set to the same potential. The pixels that are in the ideal equilibrium state exist, and also the pixels in which a potential of the connecting portion is higher than a potential of the potential barrier in the equilibrium state exist. FIG. 10 shows the potential of such pixel. As shown in FIG. 10, the charges that decide a potential of the connecting portion is decreased by ΔQ, and a potential of the connecting portion becomes higher than a potential of the potential barrier.
When the exposure is started in a state shown in FIG. 10, a part of the charges Q generated by the photoelectric converting film remains in the connecting portion. Therefore, a quantity of charges that move to the charge accumulating portion, i.e., a quantity of signals that are read out to the outside, is decreased by ΔQ, which causes a lag.
Further, the signal output characteristic with respect to a quantity of incident light depends on a potential of the connecting portion, i.e., a depth of a potential well of the connecting portion, a depth of the potential well depends on an amount of thermal diffusion of charges in the connecting portion, an amount of thermal diffusion depends on a quantity of current that flows into the connecting portion, a quantity of current that flows into the connecting portion depends on a quantity of current that flows into the connecting portion from the photoelectric converting film, and a depth of the potential well is varied along with the time. As a result, a signal output is not decided uniquely in response to a quantity of incident light, and thus it is impossible to execute a linearity correction simply based on the correction information that are held in advance.